1. Field of the Invention
The invention relates to a solenoidal magnet for producing a relatively intense magnetic field with high homogeneity in a given region of space; the invention finds its privileged field of application in an installation for forming images by nuclear magnetic resonance (NMR) and also relates, by way of application, to such an installation including this coiled magnet.
2. Description of the Prior Art
An NMR image forming installation comprises, among other things:
a magnet capable of creating a relatively intense (of the order of 0.15 to 1.5 teslas) and uniform magnetic field in an examination volume sufficiently large for receiving the patient this magnetic field is called basic field;
an assembly of gradient coils generally arranged on the same cylindrical mandrel so as to be able to generate magnetic field gradients in predetermined directions in said examination volume, during successive sequences;
radio frequency transmission-reception means for energizing the spins of the atom nuclei for which the NMR conditions are united.
The magnet is traditionally a bulky and costly assembly. Up to present two different technologies have been mainly used. on the one hand magnets with resistive coils, without pole pieces and on the other magnets with super conducting windings.
Super conducting magnets raise numerous problems.
In this system, it is of prime importance to reduce the amount of cooling fluid (liquid helium) in thermal contact with the windings so as to keep them superconducting. For this, the geometry of the coils approximates a cylinder. It is in fact known that an infinitely long solenoid generates a uniform field in its internal space and that it is possible to homogenize the field inside a solenoid of finite length, for example by modifying the axial density of turns in the vicinity of its ends. However the space requirement of the different cryogenic (liquid nitrogen and liquid helium) systems is considerable and the useful volume inside the coils is considerably reduced. In other words, for a given axis, the diameter (and so the mass) of the windings is large. Furthermore, it is imperative to be able to cope with a failure of the cooling system. In fact, any heating above a few Kelvins results in the disappearance of the super conductivity phenomenon, and the current which flowed without voltage drop in the super conducting winding, disappears suddenly as soon as said winding becomes resistive. The magnetic field disappears also very rapidly and the flux variation which results therefrom may generate induction phenomena placing the life of the patient in danger. To cope with this eventuality , a conducting cylinder (generally made from aluminium) is disposed between the coils and the patient so that the energy released by variation of the flux is dissipated in the form of eddy currents in this cylinder. Besides the fact that this cylindrical casing further reduces the useful volume, all other things being equal, its presence considerably modifies the functioning of the gradient coils. Establishment of the gradients is in fact "delayed" at each sequence by the coupling between the gradient coils and this protective casing. To overcome this new disadvantage, the power supply sources for the gradient systems must be over dimensioned.
Magnets with resistive coils which are the most frequently used include two sets of coils distributed in pairs on each side of a median plane of the examination volume. These coils often have diameters such that they are spaced apart substantially over a sphere. This design comes from the fact that it is known that a winding of turns suitably coiled at the surface of a sphere produces a uniform field in this sphere. Since a system putting this concept exactly into practice would be unusable because access to the internal volume of the sphere would be impossible, systems have been constructed comprising such sets of coils of different diameters, spaced apart along a common axis and being inscribed substantially on a sphere, the homogeneity expressed in parts per million (ppm) being obtained in a sufficient volume of the sphere by adjusting different parameters such as the characteristics of the coils, their diameters and their positions along the axis. Such homogeneous field magnets, for another application than NMR image formation, have been calculated by Garrett and the results of these calculations were published in an article in the review "Journal of applied physics" in May 1967. However, these calculations related especially to the homogeneization of the magnetic field, without proposing a structure which is industrially easy to fabricate with relatively little raw material for a given electrical power, or vice versa. The invention provides first of all a solenoidal magnet with high magnetic field homogeneity having the additional quality of a reduction of its P.M product, compared with the resistive magnet systems known up to date, M being the mass of the conductor used for producing-the magnetic field and P being the electric power consumed.